1. Field of the Invention
The present invention relates to a method for the compensation of an inhomogeneous brightness perception in scenes which are holographically reconstructed with the help of an electro-holographic device that uses a spatial light modulator means in order to reconstruct three-dimensional scenes holographically. The spatial light modulator means contains a multitude of holographic modulator cells in which a video hologram is encoded in the form of a hologram point pattern, and which are illuminated with the help of illumination means and focusing means. According to the present invention, a multitude of illumination units illuminates the light modulator means of the holographic device. In a simple case, the focusing means is a lens array with a multitude of lens elements or a lenticular array with a multitude of lenticules with cylindrical surfaces. The present invention chiefly relates to a real-time or near-real-time reconstruction of moving three-dimensional scenes with the help of holographic video means. The present invention further relates to a device with means for the elimination of brightness errors during the holographic reconstruction of scenes.
2. Technical Background
Holographic devices as understood in this invention modulate sufficiently coherent light with the help of a spatial light modulator means. The modulator cells are encoded with a hologram point data pattern and the modulator surface is illuminated with a light wave front, which is capable of generating interference, such that in a space in front of, on and behind the surface of the light modulator a spatial pattern of object light points is created by way of interference, said pattern of object light points reconstructing the optical appearance of a scene. The entirety of the light of all object light points propagates in the form of a light wave front, so that one or multiple observers can watch this light point pattern in the form of a three-dimensional scene. This means that in contrast to a stereoscopic representation, a holographic reconstruction realizes an object substitute, which is why the problems known in conjunction with stereoscopy, such as fatigue of the eyes and headache, do not occur, because there is generally no difference between watching a real scene and a holographically reconstructed scene.
The holographic device can be a holographic display, which renders visible the reconstruction in front of the eyes of one or multiple observers, or a projection device, which enlarges the reconstruction using optical means. Graphics panels, which are only a few centimetres in screen diagonal length, used in flat screen monitors and in spatial light modulators used in conventional video and TV projectors, are suitable for light modulation, for example. Known holographic devices use either transmissive or reflective light modulators.
3. Prior Art
Known devices for the holographic reconstruction of three-dimensional scenes contain optical focusing means, such as lenses, which form sufficiently coherent light, i.e. light which is capable of generating interference, into a wave, which then impinges on a transmissive spatial light modulator. The thus illuminated light modulator is encoded with a hologram and modulates the wave so as to form a wave front that carries holographic information for the reconstruction of a scene by way of interference. The light modulator thereby generates in its image-side focal plane a spatial frequency spectrum as the Fourier transform of the hologram. Such a holographic device is known for example from the international patent publication no. WO 2004/044659.
During the optical Fourier transformation of the focused light, the scene which is encoded on the light modulator is reconstructed and one or multiple virtual observer window(s) is (are) created in front of the positions assigned to the observer eyes. The size of each observer window corresponds with a period of the spatial frequency spectrum of the Fourier transform. The virtual observer window is located in the diffraction order used for the hologram. The scene is only visible in a reconstruction space through an observer window. The focusing means cover the entire modulating area of the light modulator. The light modulator can be encoded such that the reconstruction space continues behind the light modulator. An observer can thus watch the reconstructed scene in a reconstruction space which is much larger than the observer window.
Because for a large-size holographic reconstruction the light modulator is required to have a large modulating area, the lens must be of adequately large size as well. A lens with such a large area and a single optical axis can only be manufactured at great cost and effort.
In the international patent publication No. WO 2006/119920, the applicant suggests to illuminate the spatial light modulator with a light array of point or line light sources and a lens array with a multitude of lenses, e.g. a lenticular array, instead of using a single light source in conjunction with a large focusing lens. This greatly reduces the thickness and weight of the lens compared with the previously described holographic device, which minimises the costs considerably in particular for large light modulating areas, which only renders feasible the reconstruction of large-size three-dimensional scenes with the help of video holography. Each individual lens element of the lens array can be much smaller than the light modulating area, e.g. with a lens element aperture of about 10 mm. Such a lens array can be manufactured much more easily than a single large lens.
FIG. 1 shows an example of a device disclosed in WO2006/119920 and illustrates its functional principle. An array of illumination units with three coherent line light sources LS1-LS3 and lens elements 21-23 of a lens array 2, illuminates a transmissive light modulator SLM, which consists of a multitude of modulator cells. An illumination unit consists of one light source LS1, LS2 or LS3 and the nearest lens element 21, 22 or 23 of the lens array 2. The light of one illumination unit is capable of generating interference, but the light of different illumination units are not capable of generating interference with respect to each other. All lens elements 21-23 project their corresponding light source into a focal plane FP, i.e. at a defined distance to the light modulator SLM. Each lens element thereby realises a Fourier transformation. The Fourier transforms coincide and project a virtual observer window OWL/OWR in front of the left and right observer eyes, respectively.
Each illumination unit of the array thereby illuminates with a bundle of rays a separate region R1, R2, R3 on the surface of the light modulator SLM, so that all illumination units together illuminate the entire area of the light modulator in the form of a common light wave front. A common hologram sequence, which after modulation of the light wave front holographically reconstructs the moving three-dimensional scene with the help of light points P1, P2 and P3, is encoded on the light modulator SLM for the common wave front of all illumination units. As in the device described above, a holographic reconstruction 4 of the three-dimensional scene is situated between the light modulator SLM and the virtual observer window OWL/OWR. In order to ensure smooth operation, the bundles of rays from the illumination units must illuminate the surface of the light modulator SLM without gaps and without overlapping. Otherwise there may be spatial areas which are not properly illuminated and which therefore appear as dark spots in the reconstruction.
A special feature of this solution is that the modulator cells are encoded in a particular way. In contrast to the conventional encoding of holograms, the hologram information of each object light point of the scene to be reconstructed is not distributed across all modulator cells of the light modulator SLM. According to an above-mentioned patent application, the applicant suggests that, depending on the size and position of the virtual observer windows OWL/OWR, the information for each object light point is only encoded on a certain area A1, A2, A3 on the surface of the light modulator SLM. Note that the illuminated regions R1, R2, R3 do not correspond with the encoded areas A1, A2, A3.
It has been found in practice, however, that in the process of holographic reconstruction a light wave front which is based on multiple illumination units, as described above, causes a disturbed optical perception. In particular, observers perceive inhomogeneities in brightness in the reconstructed light wave front, even when the array of illumination units illuminates the light modulator SLM homogeneously. This problem was investigated extensively until a cause was found for the perceived disturbances. Finally, the cause for this problem was identified to be the interplay of the edges between adjacent lens elements of the lens array and the observer's eye pupils. Each joint, i.e. junction or boundary, between the lens elements is formed as an edge, which causes diffraction and thus disturbs the straight propagation of the light wave front towards the focal plane. As the spatial frequency distribution is filtered at the observer's eye pupils, which are located in the focal plane, not all frequencies of the spatial frequency spectrum continue to the retina of the observer's eyes. This causes the observer to perceive the reconstructed scene with an inhomogeneous brightness distribution that corresponds to the pattern of the lens array, which deteriorates the quality of the reconstructions considerably.
Diffraction at the edges of the lens elements of a lens array is particularly disturbing, because these edges lie within the reconstructed scene. If, due to the spatial frequency filtering, brightness inhomogeneities become visible at the lens margins within the reconstructed scene, this is considered to be particularly disturbing by the observers. Lens element aberrations also contribute to an inhomogeneous brightness perception.
Document WO 0075733 A1 entitled “Aberration control of images from computer-generated holograms” discloses a method for generating computer-generated hologram data for encoding the spatial light modulator of a holographic display by which aberration effects of optical components of the display are compensated. The method determines the aberrations of optical components in the holographic display and defines computer-generated hologram correction factor data for the light modulator such that the determined aberration effect is compensated. A hologram point data pattern is generated for the light modulator such that the holographic display produces a high quality holographic reconstruction.
The aberration effect of the optical components of the display for defining the computer-generated hologram is determined with the help of the optical distance of the beams through the optical components and stored in a so-called look-up table. In the above-mentioned document, imaging errors such as distortion etc. are understood as aberrations.